1. Field of the Invention
The present invention relates to a data processor for a wavemeter which executes data processing to display optical frequencies/wavelengths of optical signals measured by the wavemeter and to a data processing method for the wavemeter.
2. Description of the Related Art
In recent years, a sudden increase in data communication demands has required higher-capacity optical communications. In order to meet this demand, the Wavelength Division Multiplexing (WDM) transmission technology has come into service and has been upgraded actively. Higher-performance wavemeters are needed in order to evaluate transmission systems and optical devices utilizing the WDM transmission technology.
In a conventional wavemeter used for evaluating transmission systems and optical devices utilizing the WDM transmission technology, a series of processing steps described below are repeated sequentially: digital interference signal data obtained via A/D sampling of interference signals input from an interferometer is temporarily stored in a buffer; the digital interference data stored in the buffer is read by the Digital Signal Processor (DSP) to convert the data to frequency data; and the frequency data is further converted to optical frequency data or optical wavelength data.
Some of the conventional wavemeters utilize the fast Fourier Transform (FFT) processing when the DSP converts digital interference data to frequency data. In the FFT processing, chronological digital interference signal data is divided by predetermined period and output as frequency data.
However, as a feature of the FFT processing, discontinuous points present in chronological digital interference data cause frequency data to be displayed in the data obtained via FFT processing. To cope with this, a conventional FFT processing includes a process in which digital interference signal data prior to FFT processing is multiplied by the hamming windowing function coefficient for correcting discontinuous points.
Interpolation of discrete spectra is made to investigate detailed spectrum components of discrete spectra after FFT processing. In the interpolation, an interpolation spectrum for interpolating discrete spectra is obtained by using the optical viewpoint according to the frequency sampling theorem. Moreover, in the frequency sampling theorem, two discrete spectrum points before and after an interpolation area, that is, four points of discrete spectra in total are used to calculate an interpolation spectrum.
However, the four-point interpolation of discrete spectra after FFT processing using the conventional frequency sampling theorem has a problem because discontinuous points occur between a discrete spectrum and an interpolating spectrum.
This problem is related to an interpolation kernel obtained in the four-point interpolation. In the interpolation calculation, the interpolation calculation performed in the proximity of physically referenced discrete spectra is most seriously affected by the discrete spectra. Based on this fact, discontinuous points can be suppressed by obtaining two-point interpolation kernel from two discrete spectrum points before and after an interpolation area.
An object of the invention is to suppress occurrence of discontinuous points between a discrete spectrum and an interpolation spectrum in an interpolation area by performing two-point interpolation for obtaining two-point interpolation kernel in the interpolation of discrete spectra after FFT processing, as necessary.
According to a first aspect of the invention, there is provided a data processor for a wavemeter which processes interference signals of said optical signals in order to display optical frequencies/wavelengths of optical signals measured by the wavemeter, characterized in that the data processor comprises:
a first data conversion means for converting the interference signals to digital interference signal data (for example, A/D sampler 3 in FIG. 1),
a second data conversion means for executing the fast Fourier transform processing on digital interference signals converted by the first data conversion means to convert the digital interference signal data to frequency data (for example, DSP#A 5 in FIG. 1), and
an interpolation means for executing operation to interpolate the frequency data depending on the trend of variation in the frequency data converted by the second data conversion means and for interpolating and outputting the frequency data by using the interpolation data thus obtained (for example, DSP #A6 in FIG. 1).
According to the first aspect of the invention, in a data processor for a wavemeter which executes data processing to display optical frequencies/wavelengths of optical signals measured by the wavemeter, the first data conversion means converts the interference signals to digital interference signal data, the second data conversion means executes the fast Fourier transform processing on digital interference signals converted by the first data conversion means to convert the digital interference signal data to frequency data, and the interpolation means executes operation to interpolate the frequency data depending on the trend of variation in the frequency data converted by the second data conversion means and interpolates and outputs the frequency data by using the interpolation data thus obtained.
According to a second aspect of the invention, there is provide data processing method for a wavemeter which processes interference signals of said optical signals in order to display optical frequencies/wavelengths of optical signals measured by the wavemeter, characterized in that the data processing method comprises:
a first data conversion step for converting the interference signals to digital interference signal data,
a second data conversion step for executing the fast Fourier transform processing on digital interference signals converted by the first data conversion step to convert the digital interference signal data to frequency data, and
an interpolation step for executing operation to interpolate the frequency data depending on the trend of variation in the frequency data converted by the second data conversion step and for interpolating and outputting the frequency data by using the interpolation data thus obtained.
According to the second aspect of the invention, a data processing method for a wavemeter which executes data processing to display optical frequencies/wavelengths of optical signals measured by the wavemeter comprises
a first data conversion step for converting the interference signals to digital interference signal data, a second data conversion step for executing the fast Fourier transform processing on digital interference signals converted by the first data conversion step to convert the digital interference signal data to frequency data, and an interpolation step for executing operation to interpolate the frequency data depending on the trend of variation in the frequency data converted by the second data conversion step and for interpolating and outputting the frequency data by using the interpolation data thus obtained.
Thus, two-point interpolation using two-point interpolation kernel can be executed in the proximity of frequency data to be interpolated in an interpolation area, and four-point interpolation using four-point interpolation kernel can be executed in the other areas, as necessary. As a result, the interpolation spectrum shape can be continuously output while avoiding the interpolating spectrum from being negative, thus smoothing the details of spectrum shape to be displayed.